The invention relates to the field of industrial instrumentation and controls.
Industrial process control systems consist of inputs and outputs where the inputs are signals sent from field devices (sensing elements) to an indicator or controller corresponding to a sensed process operating parameter or variable (pressure, temperature, liquid level, differential pressure, chemical composition, and the like) and the outputs are signals sent from a controller to a final control element which, by its action corresponding to an output signal, will maintain or modify the process operating parameter or variable.
Frequently there is a need to migrate a final control element from one controller to another while the process unit is in operation without causing disruption to the operating variable (a “hot cutover”). Historically, this has been effected by one of three principal methods: (1) Utilizing block and bypass valves to allow a field operator to control the operating parameter manually while receiving radio or other communication from the central control room operator, while the control circuitry is being migrated from one controller to another; (2) using a pneumatic “boondoggle,” a tee and valve configuration that blocks the pneumatic signal of the current to pneumatic transducer that is an always-present part of the final control element where this configuration pneumatically emulates the bypass arrangement described in (1) above; or (3) using a mechanical stem clamp to force the valve to remain in its “last” position while the circuitry is being migrated. The first two methods allow some adjustments to the final control element while the electronic migration is being done, albeit being cumbersome in that the methods require verbal communication between the control room operator and the field operator. The third method allows no adjustment to the final control element whatsoever.